Metal finishing Q&As since 1989

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How does salt spray test equilibrate to normal aging process

How does salt spray test equilibrate to normal aging process and is there any faster way to do this test or any alternative test?

Azman bakri
- shah alam,selangor, Malaysia
2000

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First of two simultaneous responses--

Salt spray testing cannot be directly correlated to real life. People have tried and have been unsuccessful. The best you can do is to set a standard on how well your part should last in salt spray, and assume that if the part does well in salt spray, then it will do well in real life.

There is no accurate and repeatable test that is faster. There are different types of corrosion testing, but usually they are chosen for specific types of plating. Besides salt spray there is: CASS testing, cyclic/complex corrosion test methods, sandbox testing, and SWAT/Kesternich testing.

I am sure if anyone had a quicker version of salt spray, everyone would be doing that test instead of this one. The big problem with accelerating corrosion testing is that the quicker you go, the less you represent the true chemical reaction of rusting.

Tim Neveau
Rochester Hills, Michigan
2000

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Second of two simultaneous responses--

The salt spray test ( ASTM B117 ) is a process control tool. It does not, in itself, have a correlation to a product's service life. There are other tests which incorporate a salt spray cycle along with humidity, drying, etc. which have been found to provide correlation to field exposure. One of these tests is SAE J2334. There is substantial literature available on this test and its development.

Cynthia L. Meade
- Sylvania, Ohio, USA
2000

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Hi Azman. You got good advice from both sides. Cynthia is no doubt correct that the cyclic test is an improvement, but Tim is right that it still will not correlate well.

Consider hot-dip galvanized coatings: no finish in the world will outlast them (75 years with no maintenance is not uncommon), yet other finishes easily outperform them in accelerated corrosion tests. Why? Because the corrosion products themselves are an essential component of the corrosion resistance. Galvanized coatings slowly react with the small amount of carbon dioxide in the atmosphere over the course of months, years, and decades to form a glassy and impermeable zinc carbonate surface layer that is outstanding at blocking further corrosion. But in an accelerated corrosion chamber, those corrosion products can't form, so you aren't testing the real-world performance of the galvanized coating.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
January 9, 2013